An ultrasonic diagnostic apparatus is used to make a noninvasive checkup on a subject by irradiating him or her with an ultrasonic wave and analyzing the information contained in the echo signal, which is obtained as an ultrasonic echo from the subject. For example, a conventional ultrasonic diagnostic apparatus that has been used extensively converts the intensity of the echo signal into its associated pixel luminance, thereby presenting the subject's internal structure as a tomographic image. In this manner, the internal structure of the subject can be known.
Meanwhile, some people are attempting recently to track the motion of a subject's tissue more precisely and evaluate the strain and the elasticity, viscosity or any other physical (attribute) property of the tissue mainly by analyzing the phase of the echo signal.
Patent Document No. 1 discloses a method for tracking a subject's tissue highly precisely by sending out ultrasonic waves toward the same site of the subject a number of times at regular intervals to obtain multiple echo signals, calculating the instantaneous displacement of a local region of the subject based on a phase difference between the echo signals, and then adding the displacements together. Hereinafter, the subject tissue tracking method disclosed in Patent Document No. 1 will be described with reference to FIG. 5.
Suppose ultrasonic pulses are sent out toward the same site of the subject at intervals ΔT and ultrasonic echoes received are converted into electrical signals to obtain received signals y1(t), y2(t), . . . and yn(t), where 1, 2, . . . and n indicates the order of transmission and reception that have been repeatedly performed at the intervals ΔT and t indicates the time of reception with respect to the time of transmission which is represented by zero. In this case, the distance x from the source of reflection in the subject to the probe (which will be simply referred to herein as a “distance”) and the time t of reception of the received signal with respect to the time of transmission satisfy the following Equation (1):t=2x/C  (1)where C represents the acoustic velocity of the medium.
The received signals y1(t), y2(t), . . . and yn(t), which are functions of time, can be converted into y1(x), y2(x), . . . and yn(x), which are functions of distance, by Equation (1). That is to say, the echo signal that has been reflected from the source of reflection, which is located at the distance x from the probe, is received in the amount of time t that is represented by Equation (1). Suppose there is a measuring point at a distance X and the measuring point has moved ΔX parallel to the traveling direction of the ultrasonic wave during the interval ΔT. To calculate the magnitude of displacement ΔX of this measuring point during the interval ΔT, according to Patent Document No. 1, an quadrature detection is carried out on y1(x) and y2(x) using a reference signal with a frequency f as shown in FIG. 6, thereby obtaining complex received signals I1(x), Q1(x), I2(x) and Q2(x). And correlation calculation and arctangent calculation are performed on those complex received signals, thereby calculating the phase difference Δθ between y1(X) and y2(X) at the distance X. And the displacement ΔX is calculated by the following Equation (2):ΔX=−C·Δθ/4πf  (2)
And by adding the displacement ΔX that has been calculated by Equation (2) to the original measuring point X, the position X′ of the measuring point in ΔT is obtained by the following Equation (3):X′=X+ΔX  (3)
By repeatedly making this calculation, the measuring point on the subject can be tracked. As shown in FIG. 5, supposing the signal received next to y2(X) is y3(X), by substituting the phase difference Δθ′ between y2(X′) and y3(X′) into Equation (2) and by substituting the displacement ΔX′ obtained into Equation (3), the position X″ of the measuring point in 2ΔT can be obtained.
Patent Document No. 2 further develops the method of Patent Document No. 1 into a method of calculating the elasticity of a subject's tissue (e.g., an arterial wall, in particular). According to this method, first, an ultrasonic wave is transmitted from a probe 101 toward the vascular wall 302 of the subject 301 as shown in FIG. 7(a). And the echo signals, reflected from measuring points A and B that have been set on the same acoustic line on the vascular wall, are analyzed by the method of Patent Document No. 1, thereby tracking the motions of the measuring points A and B. FIG. 7(b) shows the tracking waveforms TA and TB of the measuring points A and B along with an electrocardiographic complex ECG. As shown in FIG. 7(b), the tracking waveforms TA and TB have the same periodicity as the electrocardiographic complex ECG, which indicates that the artery dilates and shrinks in sync with the cardiac cycle of the heart. More specifically, when the electrocardiographic complex ECG has outstanding peaks called “R waves”, the heart starts to shrink, thus pouring blood flow into the artery and raising the blood pressure. As a result, the vascular wall is dilated rapidly. That is why soon after the R wave has appeared on the electrocardiographic complex ECG, the artery dilates rapidly and the tracking waveforms TA and TB rise steeply, too. After that, however, as the heart dilates slowly, the artery shrinks gently and the tracking waveforms TA and TB gradually fall to their original levels. The artery repeats such a motion cyclically.
The difference between the tracking waveforms TA and TB is represented as a waveform W showing a variation in thickness between the measuring points A and B. Supposing the maximum variation of the thickness change waveform W is ΔW and the reference thickness between the measuring points A and B during initialization (i.e., the end of the diastole) is Ws, the magnitude of maximum strain ε between the measuring points A and B is calculated by the following Equation (4):ε=ΔW/Ws  (4)
As this strain is caused due to the difference between the blood pressures applied to the vascular wall, the elasticity E between the measuring points A and B is given by:E=ΔP/ε=ΔP·Ws/ΔW  (5)where ΔP is the blood pressure difference at this time.
Therefore, by measuring the elasticity E for multiple spots on a tomographic image, an image representing the distribution of elasticities can be obtained. If an atheroma 303 has been created in the vascular wall as shown in FIG. 7(a), the atheroma 303 and its surrounding vascular wall tissue have different elasticities. That is why if an image representing the distribution of elasticities is obtained, important information can be obtained in inspecting the attribute of the atheroma (e.g., how easily the atheroma may rupture, among other things).
FIG. 8 schematically illustrates exemplary results of calculations that have been made to evaluate the elasticity of a vascular wall by the method disclosed in Patent Document No. 2. On the monitor (i.e., on the paper on which FIG. 8 is drawn), displayed is the vascular wall's monochrome tomographic image 200. A region of interest ROI has been set on the vascular wall portion and a two-dimensional elasticity image 201 representing the distribution of elasticities in a part of the region of interest ROI corresponding to the vascular wall is superimposed in colors on the tomographic image 200. The monochrome tomographic image 200 is displayed at monochromatic gray scales corresponding to the reflection intensities along with a scale 202 indicating the reflection intensities. On the other hand, the elasticity image 201 is displayed in color tones corresponding to the elasticity values along with a scale 203 indicating the elasticity values. Also displayed under the monochrome tomographic image 200 is a biomedical signal waveform 204 such as an electrocardiogram.
However, not all of these tissue attribute values thus obtained are reliable ones but some of them are quite unreliable or inaccurate. Such values would have been obtained probably because the same tissue could not be tracked accurately or its attribute could not be evaluated accurately due to the occurrence of some noise during the measurement, the shift of the measuring point away from the acoustic line, the influence of speckle, and the ringing echo from an intense reflection source.
For example, an atheroma is included within the region of interest ROI shown in FIG. 8. It is known that an atheroma has a hard skin and a soft content. However, the two-dimensional elasticity image 201 shown in FIG. 8 indicates that there is an abnormally soft portion such as liquid and an extraordinarily hard portion that hardly deforms. And those portions indicate clearly wrong results of measurement.
To cope with such problems, Patent Document No. 3 discloses an ultrasonic diagnostic apparatus including display value evaluating means for evaluating the display value of a strain elasticity image generated based on various kinds of data that have been generated while the strain elasticity image (corresponding to the tissue attribute value) is produced. According to Patent Document No. 3, based on the results of evaluation obtained by the display value evaluating means, image information that has been graded with the strain elasticity value is displayed in an area with display value, while either the same piece of image information or a strain elasticity value is displayed in an area with no display value. According to this method, elasticity images with low degrees of reliability can be eliminated and only elasticity images with high degrees of reliability can be displayed.
On the other hand, according to Patent Document No. 4, the magnitude of shrinkage or expansion of the object of measurement between two normal end points is calculated based on the ordinary velocity (or displacement) between those two normal end points, thereby obtaining elasticity (corresponding to a tissue attribute value). That is to say, data with low reliability or low accuracy are eliminated right after the displacement has occurred and the tissue attribute value is calculated based on only displacement data with high reliability. According to this method, in regions from which such data with low reliability have been eliminated, the vertical resolution certainly decreases but accurate tissue attribute value can be obtained eventually.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2000-229078        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2005-118152        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2004-159672        